The military must put "maximum pressure" on the Taliban to clear them out of Afghanistan following David Cameron's comments about wanting British troops to leave before the next general election, a former head of the Army said.
Sir Richard Dannatt said it was important the Taliban were not able to "sit out" the time until international forces are withdrawn.
He said counter-insurgency operations "always take time" and it was important that the military effort was properly resourced and given political support.
Sir Richard said Mr Cameron had made clear "we don't want to be there for one day longer or with one soldier more than we need before we have achieved the aim".
He told BBC1's Andrew Marr Show: "This notion of five years is important, it's actually quite a long length of time.
"So that time element is an important factor but the really important thing is that we resource properly, we put the maximum amount of effort in as a coalition, as an alliance, to make sure that we succeed and do that as quickly as possible in our own interests of course, but in the interests of Afghanistan and particularly in the interests of our Armed Forces as well."
He continued: "From now on, and we've been doing it for the last little while, we must put maximum pressure on to succeed so that the Taliban don't have that sort of option to say 'we'll sit them out' for five years, 10 years or whatever.
"What's got to be remembered is these complex and difficult counter-insurgency campaigns always take time. There's a notion of strategic patience."
Put to him that Mr Cameron's comments were "unhelpful" because it gave the Taliban a timescale to work to, Sir Richard said: "I'm not going to say that because I don't think that's relevant."